Add chained swizzle assignment tests

[sudonatalie]

Add validation and execution tests for chained swizzles on the LHS of an assignment. These are valid with the swizzle_assignment language feature as long as the final indices actually assigned are free from duplicate targets.

These have been tested against the Dawn implementation in: https://dawn-review.googlesource.com/c/dawn/+/327796

---

Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Tests avoid over-parameterization (see case count report).

When landing this PR, be sure to make any necessary issue status updates.

[github-actions]

Results for build job (at b697f76):

-webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_vars:* - 21 cases, 168 subcases (~8/case)
+webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_vars:* - 23 cases, 184 subcases (~8/case)
+webgpu:shader,validation,statement,swizzle_assignment:chained_swizzle:* - 8 cases, 8 subcases (~1/case)
-TOTAL: 285319 cases, 2346405 subcases
+TOTAL: 285329 cases, 2346429 subcases